Does Size Matter?: What Your Material Composition May Not be Telling You

When attempting to anticipate the results of a processing system, a detailed analysis can give the equipment manufacturers a much better picture of the anticipated materials and how to design and organize the equipment for the highest and most efficient recovery of recyclables.

Bradley Kelley

The equipment used in today’s automated waste processing systems are state-of-the-art machines that can separate a myriad of recoverable materials from a multitude of input streams, including Single Stream (SS) recycling, Municipal Solid Waste (MSW), and Construction and Demolition (C&D) materials. However, the actual mechanics of most of this equipment separates materials by size and shape, rather than by composition. The exception is optical sorters, which do sort by material type, but even those have a size range within which they optimally perform. While a material composition report may tell a potential developer or equipment manufacturer what is in the incoming feedstock, it doesn’t tell the whole story. The size of the material components will frequently dictate where in the process flow the commodity will go, and ultimately whether or not the commodity can be recovered.

Clogging

Screening equipment is generally sized based on the material throughput and the expected material components, usually estimated from a composition report. Unless otherwise specified, assumptions are made about the individual material components and the rest of the processing system is designed with those assumptions in mind. However, if even one component is significantly different than the assumed “norm,” it can adversely affect the entire system. For instance, if there is an abundance of oversized material, it can clog the infeed equipment. Conversely, an abundance of undersized material can pass through the initial screens and clog the downstream equipment.

As a hypothetical example, say a SS material composition has 20 percent cardboard (OCC), and the equipment manufacturer designs the system assuming the majority of this commodity will be recovered by a large screen early in the process. However, in this case, the SS comes from a curbside collection using 18-gallon bins, and the majority of the generators tend to tear and cut the cardboard to fit into the small bins along with the other recyclables. In this scenario, most of the OCC would actually fall through the large screen and be carried along with the rest of the undersized stream. This means the next screen will have a higher percentage of OCC to deal with. To prevent clogging, the overall throughput of the system needs to be reduced. The downstream fiber lines may also need additional Quality Control staff to remove the OCC from other fiber streams. There can be a cascading effect throughout the processing system.

Material Composition

It is important to note that the composition should be studied in the state the input material will have upon entering the processing equipment. This is most prevalent in C&D applications, where construction debris may be unloaded in all shapes and sizes. The initial handling via loader, grapple and even dozer may help to reduce the material to a more manageable size. Some systems even employ a size reduction shredder to make the material more homogeneous in size for the downstream processing and recovery equipment. While this handling or shredding may be necessary for the processing equipment, it also produces more fine particles and small objects from the otherwise presumed recoverable commodities. Studying the material composition and size as it’s loaded into the system will give the best indication of what can be recovered, and the system designers and operators may use this knowledge to indicate if more or less handling could improve recovery and efficiency.

MSW may be the most critical to understand the sizing, as it is very difficult to make assumptions on the type and size of the commodities that may be in the trash. Repeated handling may also greatly change the nature of shape and size of this material stream. Most Mixed Waste Processing (MWP) systems have equipment to mechanically open plastic trash bags prior to the screening equipment and although the size of the unopened bags should be noted so that the bag opener can be properly sized, the composition should be indicative of the material immediately after the majority of the bags are opened. A composition report for MSW that includes sizing would enable operators and designers to better predict which commodities could be recoverable and what equipment would be best used for that recovery.

The goal of sizing the material is to help predict where the commodities may end up in the process flow. For example, an aluminum can won’t be recovered unless it goes over the Eddy Current Separator (ECS) unit or is manually retrieved. I was reminded of this recently during my company’s (GBB) quarterly street clean-up, where I noticed several aluminum cans that had been run over with a lawn mower, leaving quite a few mangled small pieces of aluminum strewn about. As I gathered these up, I thought to myself that this certainly was recyclable material, but if it were to go through an MWP system, these small pieces of aluminum would, in all likelihood, end up in the fines and not in the container stream for recovery by the ECS. There may be cases where there are a lot of recyclable metals in the fines, and an additional magnet and ECS unit may be economically advantageous for that stream.

Even equipment that doesn’t sort material by size, such as air classifiers and optical units, still performs best within certain size ranges. For an air classifier, the density and wind drag of an object comes into play in the material sorting, and the size ratio is key. Items that are much smaller or larger than the majority of the material may end up in the opposite stream from what’s expected because of size and drag within the classifiers’ air stream. With most optical units, a size range is required for accurate detection and appropriate trajectory characteristics. If the object is too small, it may not be detected or not fly the proper trajectory in the ejection chamber. Conversely, if the object is too large, the air jets may not be able to change the trajectory enough for recovery, or the object may not even fit the optical unit housing.

Material Sizes

So what material size ranges should be considered? The answer is subjective, and equipment manufacturers would probably all have different answers. Obviously, too many size ranges are not practical to try and sort during a composition study. However, even a few size ranges would still allow for some interpretation to better understand how the material will behave in a processing system.

Fines

“Fines” may be one of the few size allocations that are already frequently interpreted in material composition studies, although the composition can be a variety of materials depending on the type of stream. In MSW streams, fines are composed of many materials including broken glass, food waste, soil and rocks, and even some paper and plastics. Except for specialized applications, most fines screens will remove material less than 2” to 3” in size. For a material composition, this should be the range that would define a “fine.”

4” to 8”

The next size range would likely be disputed among manufacturers, as most have a multitude of screen opening sizes in the 4” to 8” range. For simplicity sake, and maybe a compromise, a 6” square seems reasonable as a sizing template. For MSW and SS, most containers, metal cans and some of the fiber are smaller than a 6” square, while most fiber and film will be larger than this size. Defining material to this template size would still give an indication to the screen manufacturers of how the material stream will behave, especially with regard to fiber. If a material composition indicates a great deal of the fiber is less than 6”, or if almost none is, this may impact the screen openings that are used and potentially the overall screen size.

12” to 20”

The next size designation is also likely up for debate. This would help define items that would frequently be “the overs” on a large screen such as one for OCC with openings that may be from 12” to 20” or more. Again, a compromise might be a 16” square template for sizing, and the manufacturers could interpolate from there. Another factor that comes into play is the type of material. For instance, most full-size old newspaper (ONP) would be larger than our 16” square, but would likely still fall through the openings in a large screen due to its physical characteristics. This could be acceptable as these characteristics are well known. However, for a few items there may need to be some distinction, such as with film. A heavy film will act more like OCC on a screen while thin film will behave much more like ONP. Differentiating such items within the material composition is very helpful to understanding the overall flow characteristics.

Oversized Items

The last size to consider would be “oversized items,” or materials that could jam or damage the downstream equipment. The allowable input size to the processing system can vary depending on the equipment and the material. For example, C&D systems are generally set up to deal with much larger components than an SS or MWP facility. In the same vein, large OCC from a commercial source may also be considered oversized, but would be expected at an SS or MWP facility that accepts commercial recycling. However, nearly everyone would agree that a kiddie pool or car bumper would constitute an oversized item. This is important to know as these items need to be manually removed, either from the tip floor or at the pre-sort station prior to the initial processing equipment.

Logistically, one of the easiest ways to size items during a material composition study is to have a template with outlines of the different sizes. Several may be necessary so that all sorters have easy access to them. As items are cataloged by type, they can also be sized via the template. Obviously some interpretation will be required, as few items are perfectly square in shape. The overall area of an item is also a factor in the screening, so one way to help with the interpretation is to look at the item over the template, and if the object is longer but narrower than the template square, imagine that it was folded so that it’s much closer to a square shape. If it would then fit within the square, it could be considered to be under that size, but if it were folded and still larger than the template, than it would be considered in the next size range. It would take some practice, but mental “folding” is a way to visualize where an item should be cataloged. This would not apply to the largest items. Any object that cannot fit within the oversize template in either length or width should be considered oversized.

A Much Better Picture

When attempting to anticipate the results of a processing system, a detailed analysis like this would give the equipment manufacturers a much better picture of the anticipated materials and how to design and organize the equipment for the highest and most efficient recovery of recyclables. The size of the items in the material stream will greatly dictate whether the desired recyclable items will end up in a stream where they can be recovered. The size of the components can also affect how efficient and functional the equipment will process the material streams. Knowledge of this before the processing system is actually designed, built and placed into operations can be very helpful in getting the right system for your material, and so the operator can recover the most recyclables in the most efficient manner. A detailed material composition that includes size ranges would certainly be a useful tool to aid equipment manufacturers in designing their systems.

Bradley Kelley is a Senior Project Engineer with Gershman, Brickner & Bratton, Inc. (GBB) in Fairfax, VA. He has nearly 20 years of experience in Mechanical and Systems design including waste processing, wood waste and biomass and aggregate recovery. Bradley can be reached atbkelley@gbbinc.com.

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